Stack Unwinding in C++

The process of removing function entries from function call stack at run time is called Stack Unwinding. Stack Unwinding is generally related to Exception Handling. In C++, when an exception occurs, the function call stack is linearly searched for the exception handler, and all the entries before the function with exception handler are removed from the function call stack. So exception handling involves Stack Unwinding if exception is not handled in same function (where it is thrown).

In the above program, when f1() throws exception, its entry is removed from the function call stack (because it f1() doesn’t contain exception handler for the thrown exception), then next entry in call stack is looked for exception handler. The next entry is f2(). Since f2() also doesn’t have handler, its entry is also removed from function call stack. The next entry in function call stack is f3(). Since f3() contains exception handler, the catch block inside f3() is executed, and finally the code after catch block is executed. Note that the following lines inside f1() and f2() are not executed at all.